A material (thermal conductive sheet), obtained by curing a resin composition prepared by blending a thermal conductive filler for improving thermal conductivity, such as alumina and silica, into a flexible resin so as to form a sheet shape, has been used for an application for radiating heats generated in the electric/electronic part and the like by interposing between a heat-generating body, such as an electric/ electronic part, installed in various electric products such as personal computers and plasma displays, and a heat-radiating body, such as a heat sink, a heat-radiating fin and a metal heat-radiating plate. Generally, in most cases, the surfaces of the heat-generating body and the heat-radiating body are not smooth so that, in order to increase the contact area with them to enhance the thermal conductive efficiency from the heat-generating body to the heat-radiating body, the thermal conductive sheet is required to have flexibility.
Conventionally, silicone rubber and silicone gel have been used as resins having flexibility; however, problems have arisen in which these resins are expensive, the curing process requires a long period of time to cause deterioration in productivity and siloxane having a low molecular weight is generated to cause a contact failure in the electronic parts.
In order to solve the above-mentioned problems, a heat-radiating (thermal conductive) sheet, which is formed by curing a binder containing a copolymer having a glass transition point of −30° C. or less and a monomer having an unsaturated bond, has been proposed (for example, Japanese Patent Application Laid-Open No. 2001-335602). Moreover, aside from this, a non-silicon-based resin composition for a heat-radiating material, which contains a polymerizable monomer mainly composed of an acrylic acid ester monomer having an alkyl group with carbon atoms in a range from 2 to 18, a photopolymerization initiator and a thermal conductive filler, has been proposed (for example, Japanese Patent Application Laid-Open No. 2002-155110). In these inventions, however, although the problem of the contact failure in electronic parts due to generation of siloxane having a low molecular weight is solved, since neither silicone rubber nor silicone gel is used, it is found that the moldability at the time of molding the thermal conductive sheet and the flexibility of the resulting sheet need to be improved, as a result of examinations conducted by the inventors, etc. of the present invention.
Moreover, a thermal conductive sheet, which contains a high-molecular gel such as acrylic gel, a thermal softener that is in a solid state or in a paste state at normal temperature and becomes liquid when heated and a thermal conductive filler (filler agent) has been proposed (for example, Japanese Patent Application Laid-Open No. 2002-234952). However, even this thermal conductive sheet has some room for improvements in flexibility.
Furthermore, an acrylic urethane resin has been known in which an acrylic oligomer having two hydroxyl groups in one molecule and a polyfunctional isocyanate having at least two isocyanate groups in one molecule are used as the resin composition (for example, Japanese Patent Application Laid-Open No. 2002-30212).
In the case when these resins are used for manufacturing thermal conductive sheets, the following problems tend to be raised: the resulting composition has high viscosity to cause deterioration in operability; since it is not possible to blend a thermal conductive filer at a high blending rate, the thermal conductive property of the resulting thermal conductive sheet becomes low; it takes long to disperse the thermal conductive filler uniformly in a resin by kneading, resulting in deterioration in productivity; and since upon manufacturing a composition, the composition is poor in defoaming property for eliminating air mixed therein, bubbles tend to generate in the resulting heat-radiating sheet to cause deterioration in the moldability and thermal conductive performance.
Moreover, for example, a resin composition which has a urethane bond and is used for coating, and which contains a metal organic compound serving as a urethane reactive catalyst and an acidic substance, has been known (for example, Japanese Patent Application Laid-Open No. 2001-240797). However, this document has no description concerning a thermal conductive material, and with respect to the addition of an acidic substance, only its effects as a technique for prolonging the usable time of a coating composition to be cured by using a urethane crosslinking reaction are described therein. Moreover, this document also has no description with respect to a hardness reduction that occurs when the composition is exposed to high temperatures for a long period of time.
Here, (meth)acrylic resins have been widely used as a base resin for a thermal conductive material (Japanese Patent Application Laid-Open No. 2003-49144, the above-mentioned Japanese Patent Application Laid-Open No. 2002-155110, Japanese Patent Application Laid-Open No. 11-269438, etc.); however, in most cases, these (meth)acrylic resins are prepared in the form of a liquid-state resin composition (referred to as acrylic syrup) in which a (meth)acrylic polymer and a monomer are mixed (for example, Japanese Patent Application Laid-Open No. 9-67495, etc.), and the user further subjects this liquid-state resin composition to a thermal polymerization (radical polymerization) process or a crosslinking (curing) process with a crosslinking agent to form a net-work structure so that a final (meth)acrylic resin product (a molded product, a sheet, etc.) is formed. Here, a method has also been known in which a (meth)acrylic resin is manufactured by using a special catalyst (Japanese Patent Application Laid-Open No. 2000-128911, etc.)
With respect to acrylic syrups of this type, those syrups have been known in which, for example, a (meth)acrylic monomer is polymerized in toluene, and after toluene has been removed, a (meth)acrylic monomer is added thereto to form a liquid-state resin (syrup) and the syrup is polymerized by using a polymerization initiator (see Examples 7 to 9 in the above-mentioned Japanese Patent Application Laid-Open No. 9-67495). Moreover, the above-mentioned Japanese Patent Application Laid-Open No. 2003-49144 has disclosed a method in which a (meth)acrylic monomer is radical-polymerized in ethyl acetate, and after the resulting matter has been applied on a PET film, the PET film is dried. However, the (meth)acrylic resins, obtained by these methods, contain a solvent (toluene, ethyl acetate, etc.). In other words, although these methods remove the solvent in the middle of the processes, it is impossible to remove the solvent completely. When the (meth)acrylic resin thus obtained is used, there is a possibility that the residual solvent might evaporate even little by little, and the possibility becomes greater in the case of applications (thermal conductive materials and the like) in which heat is imposed.
The objective of the present invention is to solve the above-mentioned conventional problems, and consequently to provide a resin composition for a thermal conductive material that is superior in moldability, and the thermal conductive material that exerts a superior thermal conductivity and flexibility for a long period of time.